This invention relates to the preparation of metal-matrix composite materials by a melting and mixing process, and, more particularly, to a technique for enhancing the wetting of the matrix to the particulate reinforcement.
In one approach for manufacturing composite materials, a metallic alloy is melted in a reactor, and particles of a reinforcing material are added to the melt. The metallic alloy and the particulate material are mixed under vacuum and with high shear conditions to cause the metallic alloy to wet the particles. The wetted particles are not rejected from the melt, so that the wetted particles thereafter remain distributed throughout the melt with only gentle stirring.
Upon cooling and solidification of the metal, a generally uniform distribution of discontinuous reinforcing particles is present throughout a metallic alloy matrix. Desirably, there are few voids in the composite material and little or no other reaction products. The composite material exhibits specific modulus and strength properties, as well as wear resistance, superior to those of the unreinforced matrix material, with moderately increased cost. Composite materials produced by this technique, as described in U.S. Pat. Nos. 4,759,995 and 4,786,467, have enjoyed considerable commercial success in only a few years after their first introduction.
The wetting of the molten metal to the particles is critical to the success of composite materials fabrication by this technique. If the particles are not completely wetted, a high void fraction is present, and the mechanical properties of the composite are poor.
Thus, while the described high-shear mixing process is fully operable, there is an ongoing need for a technique that would improve the degree of wetting of each particle, accelerate wetting, or ensure that all particles are fully wetted during the high-shear mixing process.
Various techniques have been proposed for improving wetting of the matrix to the particles during the mixing process. Most involve either making alloy additions to the matrix or precoating the surfaces of the particles with a layer that is more easily wetted than is the particle itself. For example, it is known that about 1 to 3 weight percent magnesium alloying content in the matrix is useful in improving wetting to oxide particles. A thin nickel coating on an aluminum oxide particle will improve wetting of aluminum alloys to the particles.
While such techniques are each valuable in certain circumstances, they limit the general applicability of the technique. There is therefore a need for an improved mixing process to achieve more complete wetting of the matrix to the particles. The present invention fulfills this need, and further provides related advantages.